Although initial work on organic light-emitting diodes (OLEDs) has focused on fluorescence emission, this type of emission is limited by the inability to capture energy from triplet excited states. Therefore, current research has been aimed towards electrophosphorescent (EP) devices, which in principle are capable of achieving up to 100% internal emission efficiency through harvesting both singlet and triplet excitons and which is potentially a more efficient approach than a purely fluorescent device where only singlet exciton provides a radiative pathway. One way of achieving electrophosphorescence involves doping heavy metal complexes (e.g., Ir and Pt complexes) into organic host materials in a multi-layered organic light-emitting diode. These heavy metal complexes typically exhibit efficient intersystem crossing from their singlet to triplet excited states and the triplet states can then relax through phosphorescence. EP devices have been demonstrated as candidates for full-color display applications. Devices have also been developed that exhibit broadband white emission. In guest-host systems where energy transfer is the predominant mechanism for generation of luminescent species, charge carrier recombination principally occurs on the host materials, and the energy created is transferred from the singlet or triplet state of the host to the singlet or triplet excited state of the phosphorescent guest. In order to develop an effective guest-host system, the host material should fulfill energy-level matching with neighboring layers of the device for efficient charge injections and with the phosphorescent guest material for effective energy transfer of singlet and triplet excitons (in particular requiring that the singlet and triplet energies of the host should be higher in energy than the those of the guest), and for efficient triplet confinement at the guest (requiring that the host triplet energy is higher than that of the guest to avoid energy transfer back from the excited guest to the host).
The development of white polymer light-emitting devices has been aimed at their potential applications in low-cost back-lighting in liquid-crystal displays, in full-color displays, and as next generation lighting sources envisioned to replace the incandescent light bulb and fluorescent lamp in some applications. White light emission in polymer light emitting diodes, which is usually difficult to achieve from a single polymer, can be sought in polymer-blend systems with multi-functional emitting components. Based on the guest-host approach, single dopant white light-emitting diodes (WOLED) have been achieved in small molecule systems.
There is currently a need to establish more efficient and cost effective methods for OLED fabrication than the widely used high-vacuum vapor-deposition methods for devices where the host and guest are small molecules.